Technique to control a piston of a cylinder apparatus having an air cushioning mechanism is disclosed in an official gazette of Japanese Patent Application Laid-open No. 2002-250307 which is a patent document. In this technique, two on-off opened/closed solenoid valves are respectively coupled to a head-side pressure chamber and a rod-side pressure chamber formed to sandwich the piston, a throttle valve and a rapid exhaust valve are connected to the rod-side pressure chamber, and these valves are caused to operate based on pressures of the head-side pressure chamber and the rod-side pressure chamber. The cylinder apparatus has the air cushioning mechanism and the valves and the piston are actuated by using compressed air as a driving source and based on pressure information. Therefore, there is no electrically controlled portion except driving of the solenoid valve and there is an advantage that it is easy to cope with a breakdown and the like. On the other hand, many valves are necessary and the number of parts increases.
As an improvement over this cylinder apparatus, the present inventor has developed a system for controlling compressed air to be supplied to a head-side pressure chamber 101 and a rod-side pressure chamber 102 by using one servo valve 100 instead of the two on-off opened/closed solenoid valves as shown in FIG. 5. In this improved cylinder apparatus, the servo valve 100 functions as a selector valve in pushing out of the piston 103 and performs a positioning function in returning of the piston 103 to thereby stop the piston 103 in a desired midpoint position. Moreover, a valve opening degree can be adjusted by using the servo valve 100 and, as a result, a flow rate can be controlled. Therefore, the piston 103 can smoothly be moved to a predetermined position.
In this improved cylinder apparatus, however, the number of parts is still large because the cylinder apparatus has an air cushioning mechanism and needs to use a throttle valve 104 and a rapid exhaust valve 105 and because a pressure setting unit 106 for supplying compressed air of a predetermined pressure to the servovalve 100 is necessary, though the two solenoid valves have become unnecessary by using the servo valve 100. Although various cylinder apparatus with simple structures using servo valves have been developed, their control mechanisms are not necessarily satisfactory and therefore high-speed and smooth control cannot be obtained.